Process for recovering styrene and xylenes from cracked oil by extractive distillation with a dealkyl acetamide

ABSTRACT

AROMATIC HYDROCARBONS AR RECOVERED FROM CRACKED OIL BY EXTRACTIVE-DISTILLATION. STYRENE, PARTICULARLY, IS DIRECTLY OBTAINED FROM THE CRACKED OIL BY EXTRACTIVE-DISTILLATION.

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GENERAL FIELD OF THE INVENTION The present invention relates to aprocess for recovering aromatic hydrocarbons contained in cracked oilobtained as by-products from the production of ethylene by thermaldegradation (cracking) of naphtha. Further, the present inventionrelates to a process for obtaining styrene and Xylene from the crackedoil directly by extractivedistillation using solvents defined hereunder.

Ethylene, being one of the most important intermediate products of thepetro-chemical industry, is produced by heat-degradation of a refinerygas, a natural gas, or a naphtha. However, cracking of naphtha is themain method for producing ethylene in most locations in the world, otherthan those having an abundance of natural gas.

'In the cracking of naphtha, naphtha is thermally cracked at about 800C., and ethylene is produced at a yield of about Ztl-35% by weight. Atthe same time, a plentiful amount of by-product called cracked oil isobtained. The oil consists of many different components, Because itcontains a large amount of unsaturated hydrocarbons such as oleins,dienes, acetylene derivatives and the like, it is very difcult toseparate and rene a specific component directly from the cracked oil.(In the present invention the term unsaturated hydrocarbons meansunsaturated hydrocarbons having an oleiinic double bond or an acetylenictriple bond, but does not mean the double 'bond of an aromatic ring.)However, as a large amount of the cracked oil is formed and it containsa large amount of aromatic hydrocarbons, it is a very important matterto utilize these aromatic hydrocarbons for reducing the cost ofethylene.

DISCUSSION OF THE PRIOR AtRT Heretofore, in aromatic hydrocarbons suchas benzene, toluene and Xylene contained in the cracked oil obtained byethylene production have been recovered by, in the first step,hydrogenating unsaturated hydrocarbons such as mono olens and dienes toform saturated hydrocarbons, and then separating aromatic hydrocarbonsby extraction. Namely, at rst, hydrocarbons having up to carbon atomsand hydrocarbons having more than 9 carbon atoms were removed bydistillation from the cracked oil, the resulting mixture washydrogenated to convert unsaturated hydrocarbons such as olens, dienesand acetylene derivatives, and then aromatic hydrocarbons were separatedfrom the mixture by extraction using solvents such as diethyleneglycol,sulfolane and N-methylpyrrolidone. Benzene, toluene, xylene and otheraromatic hydrocarbons were separated by further distillation of theresulting hydrocarbons. Benzene and toluene thus recovered did not haveany problem as to quality; however,

3,684,665 Patented Aug. 15, 1972 ice with regard to Xylene quality,about 40-50% by Weight ethylbenzene was contained in this xylene. Suchxylene might be unsuitable as a raw material for production of P-Xylene.The composition of this xylene, namely a composition of aromatichydrocarbons having 8 carbon atoms depends on the condition of thermaldegradation of the naphtha. However, ordinarily cracked oil beforehydrogenation has about 30-40% by Weight styrene instead ofethylbenzene. Styrene contained in the cracked oil is converted intoethylbenzene by hydrogenation. Moreover, attempting to obtain moreethylene by cracking of the naphtha under more severe crackingconditions has a tendency to increase the styrene content in the crackedoil. Therefore, it is diicult to obtain Xylene of good quality, namelyxylene containing high concentration of P-Xylene, from cracked oil byprior art methods. Further, in the prior art methods it is necessary touse a large quantity of hydrogen gas because of hydrogenation of thestyrene that is contained in the cracked oil.

SUMMARY OF THE INVENTION A novel and advantageous process is providedfor recovering aromatic hydrocarbons particularly styrene and xylenefrom the cracked oil, which consists of, in order to concentrate C-8aromatic hydrocarbon, separating hydrocarbons having up to about 5, morepreferably 7, carbon atoms and more than 9 carbon atoms from the crackedoil by fractional distillation, producing a fraction containing Xyleneat the top of the distillation column and a residue containing styreneat the bottom of the distillation column by extraetive-distillation,and, in necessity, hydrogenating the fraction containing the Xylene.

BRIEF DESCRIPTION `OF THE DRAWING The attached drawing is a schematicillustration of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION It is an object of this inventionto recover aromatic hydrocarbons especially Xylenes and styrene from thecracked oil obtained from the cracking of naphtha. It is also an objectof this invention to obtain Xylenes containing p-Xylene of highconcentration from a fraction of hydrocarbons having 8 carbon atomscontained in the cracked oil. It is another object of this invention toobtain styrene directly from the cracked oil. The other objects of thisinvention will appear in further detail hereinafter.

The present invention relates to a process for recovering aromatichydrocarbons from cracked oil which consists of (l) removinghydrocarbons having up to about 5, more preferably about 7, carbon atomsand more than 9 carbon atoms from the cracked oil by fractional distillation, (2) obtaining the fraction mainly consisting of Xylene fromthe top of the distillation column and the residue substantiallyconsisting of styrene and solvent from the bottom of the distillationcolumn by extractivedistillation of the middle fraction of step (l), and(3) in necessity, hydrogenating the fraction containing xylenes obtainedfrom step (2).

`In the present invention, it is sometimes preferable that the presentinvention be carried out by: (1) separating hydrocarbons having about 5carbon atoms or less from the cracked oil by fractional distillation,(2) separating benzene, toluene (as a fraction having a lower boilingtemperature than Xylenes or styrene) and hydrocarbons having at leastabout 9 carbon atoms (as a residue having a higher boiling temperaturethan xylenes or styrene) by fractional distillation, (3) obtaining thefraction of xylenes from the top of the distillation column and styrenefrom the bottom of the distillation column by extractive-distillation ofthe fraction containing xylenes and styrene obtained from step (2) and(4) hydrogenating the fraction of xylenes.

According to the present invention benzene, toluene, Xylene and styrenecan be recovered from the cracked oil eiciently and economically.

The cracked oil employed according to the present invention may consistof by-products from a plant producing ethylene by heat degradation(cracking) of naphtha at about 700-l000 C. Since the cracked oilconsists of multiple components, it is diflcult to state its compositioncritically because various compositions are created under differentcracking conditions. However, in the present invention, all cracked oilsfrom which have been removed hydrocarbons having up to 5 carbon atomsare considered to be useful. It is not necessary to remove thesehydrocarbons completely, but the total retained contents of thesehydrocarbons should not be in excess of about 1% by weight. As abovementioned, t is not possible to set forth the composition of the crackedoil critically, however, normally a cracked oil containing about 30-80%by weight aromatic hydrocarbons may be used. Of course, there is nointention to limit the present invention with respect to the contentsabove mentioned. The cracked oils are readily available commercially asby-products from plants producing olefins (ethylene) by cracking ofnaphtha.

The step of removing hydrocarbons having up to 5 (7) carbon atoms andmore than 9 carbon atoms from the cracked oil is usually carried out byfractional distillation. All of the known distillation operations areuseful in the present invention. Normally, the distillation operation iscarried out at a temperature of about 70 C., under a normal atmosphericpressure, with a reux ratio of about 2, preferably at a temperature of135 C. and 150 C. of the converted value of normal atmospheric pressure,and with a reflux ratio of 3. In this distillation procedure, twodistillation columns are preferably used.

With regard to the step of separating aromatic hydrocarbons as maincomponents from such cracked oil, namely separating the fraction havinga lower boiling temperature than Xylene or styrene and the residuehaving a higher boiling temperature than Xylene or styrene from thecracked oil, the operating condition of the distillation step is set upand controlled at the discretion of the operator. However, it ispreferable that this fraction contains a few parts of an aromatichydrocarbon having 8 carbon atoms and that the residue consists ofhydrocarbons having at least 9 carbon atoms.

In this distillation operation, when one distillation column is used,the fraction having a lower boiling temperature is recovered from thetop of the column, the fraction of desired aromatic hydrocarbons isrecovered from the middle of the column, and the residue is re- Icovered from the bottom of the column. Of course, it is possible to usetwo or more distillation columns. The distillation is carried out underatmospheric pressure or reduced pressure. Normally, the distillationoperation 1s carried out at a temperature of 70 C. under a normalatmospheric pressure, and with a reux ratio of 2. However, theseoperating conditions are not intended to limit the scope of the presentinvention.

The desired product fraction thus obtained consists of aromatichydrocarbons having 8 carbon atoms as its main components. However, asbenzene and toluene are separable easily from the fraction, it ispossible to include a great amount of benzene and toluene in thefraction, although these amounts may be greater than the amounts ofXylene and styrene. However, it is preferable that the total amount ofbenzene and toluene should not exceed 10% by weight. However, it is notdesired that any great amount of hydrocarbons having at least 9 carbonatoms be contained in this fraction, because they are an obstacle tosubsequent extractive-distillation. The total amount of thesehydrocarbons should be less than about 1% by weight.

The separation of xylene and styrene from the fraction is successfullycarried out by eXtractive-distillation. The extractive-distillation stepis carried out by feeding the fraction obtained from the above mentionedoperations into a column at the middle of the column and also feeding asolvent, hereunder dened, into the column near the top of the column,and then the fraction containing concentrated xylene is recovered fromthe top of the column and concentrated styrene is recovered from thebottom of the column with solvent. One or more distillation columns areused in the extract-ive-distillation, however, it is preferable to usetwo distillation columns.

Suitable solvents include a dialkyl acetamide such as dimethylacetamide,a dialkylsulfoxide such as dimethylsulfoxide, an alkylene carbonate suchas ethylene carbonate and propylene carbonate, a lactone such as'ybutyrolactone and methyl-fy-butyrolactone, a lactum such ase-caprolactum, phenol, an alkylphenol, a salicylic acid alkylester,aniline, an alkylaniline, a phthalic acid alkylester, a tetraalkylurea,an N,Ndialkyl carbamicester, a glycol monoalkylether such asdiethyleneglycol monoalkylether and triethyleneglycol monoalkylether andN- methylpyrrolidone.

In the above mentioned alkyl substituted compounds, it is preferable touse compounds having alkyl group of 1 to 3 carbon atoms.

One to 50 mol of solvent, preferably 2-10 mol of solvent, is used as anextractive-distillation solvent based on one mol of styrene contained inthe feed.

In the extractive-distillation step, it is preferable to use apolymerization inhibitor such as hydroquinone for preventing thermalpolymerization of styrene. Further, the temperature and pressure shouldbe controlled in the extractive-distillation step; it may be below 120C. at the bottom (boiler) of the distillation column, preferably withinthe range of about -110 C. The pressure should be determined accordingto the structure of the distillation column and the nature of thesolvent. However, the pressure may usually be within the range of about10-200 mm. Hg absolute, preferably within the range of about 20-100 rnm.'Hg absolute at the top of the column. Virtually any known inhibitor maybe used, such as hydroquinone, tert-butylcatechol, phenothiozine,sulfur, etc. The amount of inhibitor employed in the distillation stepmay be 10-500 p.p.m. based on the weight of the feed, or even more. Thepreferable reux ratio is within the range of 10-20.

It is considered surprising that styrene can be obtained from crackedoil directly according to the present invention.

In the extractive-distillation step of the present inwention, when usinga solvent selected from the group consisting of a dialkyl acetamide suchas dimethylacetamide, a dialkylsulfoxide such as dimethylsulfoxide, analkylene carbonate such as ethylene carbonate and propylene carbonate, alactone such as q/-butyrolactone and methyl-'ybutyrolactone, a lactumsuch as e-caprolactum, phenol, an alkylphenol, a salicylic acidalkylester, aniline, an alkylaniline, a phthalic acid alkylester, atetraalkylurea, an N,N-dialkyl carbamicester, a glycol monoalkylethersuch as diethyleneglycol monoalkylether and triethyleneglycolmonoalkylether and N-methylpyrrolidone styrene can be obtained at a highpurity and at a high yield ratio.

Heretofore, styrene has been produced by dehydrogenation of ethylbenzeneof high purity and distillation of the resulting products under reducedpressure. Ethylbenzene has been obtained by reaction of ethylene andbenzene, or separated from fractions of Xylenes by distillation. Itwould be thought to be particularly advantageous to recover ethylbenzenefrom the xylene fraction since great quantities of the xylene fractioncan be readily obtained from the patro-chemical industry. However, infact, because the boiling points of p-Xylene and m-Xylene are close tothat of ethylbenzene, it is sometimes necessary to use a distillationcolumn having as many as 350 plates for separating ethylbenzene from thexylene fraction. Therefore, such method has a drawback both technicallyand economically.

On the other hand, the method which consists of transformingethylbenzene contained in the xylene fraction into styrene bydehydrogenation and recovering styrene thus obtained by distillation hasalso been examined. However, in this method, Xylenes, especiallyO-Xylene (boiling point 144.4 C.) are very close to the boiling point(1\45.2 C.) of styrene, and it is virtually impossible to separatestyrene from the resulting mixture by normal distillation. Therefore,the technique of separating styrene from a xylene fraction gives anunsolved problem for practical use at this time. As to this technique,some improved methods have been proposed as follows:

(1) The method of separating styrene by extraction using a silver saltaqueous solution (2) The method of separating styrene by azeotropicdistillation (3) The method of separating styrene byextractivedistillation using dialkylforrnamide especiallydimethylformamide as a solvent (4) The method of separating styrene bypolymerization of styrene in the Xylene fraction However, these methodshave still presented problems. The method (l) is expensive because ofthe silver salt; the method (2) has a low separation etlciency; themethod (3) is troublesome because styrene and xylene respectively formazeotropics with dimethylformamide and so it is virtually impossible torecover styrene and xylene from solvent by distillation; the method (4)is actually impractical.

However, styrene can be -obtained from the xylene fraction easily by theextractive-distillation using the above mentioned solvents.

In the present invention, the residue thus obtained consists essentiallyof styrene and solvent. Styrene can be separated easily by normaldistillation of the residue under reduced pressure. It is preferablethat the pressure be controlled within the range of about 20-100 mm. Hgabsolute at the top of the column. In order to prevent thermalpolymerization of the styrene, it is preferable that the temperature bekept below 120 C. at any plate of the column. It is also preferable touse an inhibitor in this distillation as well as in theextractive-distillation. Styrene can be recovered from the top of thecolumn as a fraction.

In the extractive-distillation step, the fraction taken from the top ofthe column consists mainly of xylene but toluene, benzene and othersaturated and unsaturated aliphatic hydrocarbons may be present. Thefraction, as it is or after washing with Water, may be hydrogenated byusing a catalyst such as Mo, Co, Ni and combinations thereof. Thehydrogenation reaction may be carried out under a pressure of -200kg./cm.2 absolute and at a temperature of 10D-400 C. In thishydrogenation procedure it may be advisable to use a mixture with xylenefraction of extractive-distillation and the lower fraction and/or theresidue obtained from the distillation of the cracked oil.

According to the present invention, aromatic hydrocarbons such asbenzene, toluene, xylene and styrene can be recovered from the crackedoil successfully. Especially according to the present invention, xylenehaving good quality (containing a high concentration of P-xylene and alow concentration of ethylbenzene) can be obtained, and also styrenehaving high purity can be obtained from cracked oil directly. P-xyleneis an important raw material of terephthalic acid, and styrene is alsoimportant as a raw material in plastics. The present invention shouldcontribute to reduce the costs of these products greatly.

Some examples of the process of the present invention will now bedescribed.

Example l Cracked oil of depentanized bottom product containmg 7 .5% byweight styrene was obtained from the commercial plants of naphthacracking. Lower fractions whose boiling temperatures of up to 130 C. andhigher fractions of beyond '150 C. were removed by fractionaldistillation of said cracked oil.

The remaining oil was extractively distilled, using two distillationcolumns. The first column which has plates number of 50 was operatedunder 70 mm. Hg absolute at 60 C. using a reflux ratio of l2, and thesecond column which has plates number 60 was operated under 20 mm. Hgabsolute at 45 C. using a redux ratio of 15. The oil was fed into themiddle plate of the first column and dimethylacetamide in the sameamount of the oil (containing 300 ppm. of tert-butylcatechol) wassupplied from the top of the column. The residue from the bottom of thefirst column was fed into the middle plate of the second column, anddimethylacetamide in a quantity of `0.5 times the amount of the residuewas supplied to the top of the secondl column. The fractions from thetop of the second column were recycled to the middle of the irst column,and the mixture of styrene and the solvent was obtained from the bottomcf the second column.

Said mixture was fractionated under a reduced pressure at C. with areliux ratio of 3, styrene Was thus obtained from the top of the columnhaving 99.2% purity and 86% yield ratio based on the amount of styrenecontained in the cracked oil.

The fraction from the top of the lirst extractive-distillation columnand the lower fraction were mixed together. The mixture was washed withwater, and was hydrogenated under 40 atm. at 330 C. in the presence ofCo-Mo catalysts. Xylene contained in the reaction mixture thus obtainedwas composed of 21% by weight ethylbenzene, 18% by Weight P-xylene, 36%by weight m-xylene, and 25% by Weight O-xylene. Comparably, xylenewithout separating styrene contained 47% by weight ethylbenzene and 12%P-xylene.

Example 2 The same operation as Example 1 was carried out, except using4-methyl-y-butylolactone instead of using dimethylacetamide of Example 1as the solvent.

Styrene was recovered at 98% purity and with a 75% yield ratio based onthe cracked oil.

Xylenes contained in the reaction mixture were composed of 25% by weightethylbenzene, 17% by weight P-xylene, 34% by weight m-xylene and 24% byweight O-Xylene.

Example 3 A cracked oil containing 7% by weight styrene was obtained byfractional distillation of by-product of a plant producing ethyleneafter removing hydrocarbons having up to 5 carbon atoms. The lowerfractions having boiling temperatures of up to C. under 150 rnm. Hgabsolute were separated from the cracked oil by fractional distillationand the higher fractions having boiling temperatures of at least 80 C.under 43 mm. Hg were separated by fractional distillation. Fractions ofhydrocarbons having 8 carbon atoms as the main portion were obtained.

Said fractions were extractively-distilled. Two distillation columnswere used for extractive-distillation. The rst column, composed of 50plates, vwas operated under 50 mm. Hg absolute at 55 C. using refluxratio of 15, and the second column, composed of 60 plates was operatedunder 20 mm. Hg absolute at 45 C. using a reflux ratio of v20.

Said fractions were fed into the middle plate of the first column anddimethylsulfoxide (containing 500 p.p.m.

in cracked oil. The fractions from the top of the rst column werehydrogenated under 50 atm. pressure at 380 C. in the presence of Co-Mocatalysts. The xylene fraction thus obtained was composed of 20% byweight ethylbenzene, 18% by weight P-xylene, 36% by weight rnxylene and26% by vweight O-xylene.

On the other hand, the xylenes contained in the reaction mixturehydrogenated without separating styrene by extractive-distillationcontained 48% by weight ethylbenzene and 12% by weight P-xylene.

Example 7 The same operation of Example 6 was applied except using'y-butylolactone instead of dimethylacetamide as the solvent.

Styrene was recovered with 99.2% purity and at an 85% yield ratio.

Xylene contained in the reaction mixture were composed of 21% by weightethylbenzene, 18% by weight P- xylene, 36% by weight m-xylene and 25% byweight O-xylene.

Examples 8-24 Styrene and xylene were separated from each other and fromvarious compositions consisting of styrene and xylene byextractive-distillation using various solvents. In theseextractive-distillations, each distillation column had a capacity of 2.5meters height and 30 mm. inner diameter,` and was filled up with 3 x 3mm. wire nettings.

Each composition was supplied from a feed plate set up at 1.8 meterheight of each column, and also each solvent was supplied from a feedplate set up at 2.4 meters height of each column.

Xylene was recovered from the top of the column as a fraction, andstyrene was recovered from the bottom of the column as residue.

These examples produced results as shown in table.

In this table, each percentage of purity of hydrocarbon represents theconcentrate ratio except of solvent.

To be sure, in these examples, solvents were mainly recovered from thebottom of the columns together with styrene as residue.

We claim:

1. The method for separating xylene and styrene from a C3 fractioncontaining primarily xylene and styrene 10 comprising extractivelydistilling said C3 fraction with a dialkyl acetamide; recovering axylene fraction in the form of a vapor, removing the styrene in asolvent solution of the dialkyl acetamide and rectifying the solventsolution to separate the styrene from the dialkyl acetamide.

2. The method according to claim 1 wherein said extraction distilling isconducted in the presence of an eective amount of an inhibitor sucientto prevent polymerization of said styrene in said solvent solution.

3. The method according to claim 1 wherein 1-50 mols of said dialkylacetamide is used per mol of styrene in said C3 fraction.

4. The method according to claim 1 wherein said dialkyl acetamide isdimethyl acetamide.

5. The method according to claim 2 wherein said inhibitor is a memberselected from the class consisting of hydroquinone, tert-butylcatechol,phenothiozine, sulfur and mixtures thereof.

6. The method according to claim 1 wherein said xylene containingfraction is thereafter hydrogenated whereby aliphatic unsaturatedimpurities are hydrogenated and thereafter separating said impuritiesfrom said xylene.

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